It's been 69 years since the metropolitan area was hit by a late-season hurricane. Sandy's expected turbulent merger with a cold front moving in from the west, and a southern dip in the jet stream from Canada, will make it a hybrid storm, an even rarer occurrence, experts say.

The resulting storm, falling right before Halloween, has been dubbed 'Frankenstorm.'

Hurricane season lasts until the end of November. But "for Long Island, our peak tropical storm season is from late August to mid-September," said Brian Colle, a professor at Stony Brook University's School of Marine and Atmospheric Sciences.

National Weather Service meteorologist David Stark in Upton said "having a hurricane or tropical storm move along the East Coast is not uncommon in October," but usually they move offshore before they reach the New York area.

"The big difference here is we have a cold front to the west that's associated with a fairly strong jet stream in the upper atmosphere," Stark said.

"The trough with the jet stream is going to help pull Sandy up the coast and allow the two systems to interact. Rather than having Sandy go out over the ocean, it's going to help pull it back towards the coast, and then they will join when they get here and become one system."

By the time Sandy gets close to New York, most of its energy will be coming from the jet stream, not warm southern waters, Stark said.

"At this point it really doesn't matter" whether the eye of Sandy strikes Long Island "because the area of fairly intense winds is going to be over a 100- to 200-kilometer [60- to 120-mile] region," Colle said.

And while the storm won't be a nor'easter, it will mimic the duration and wind direction of a nor'easter.

"A hurricane can be in and out in 12 hours," Colle said. "This one we're talking perhaps longer than 24 hours where we have potential for damaging winds."

If the forecast holds, the hybrid storm will have prolonged winds flow from the east that will push waves and high tides onto eastern-facing beaches, particularly along Asharoken and Bayville on Long Island Sound, South Shore areas such as Lindenhurst, and New York City.

"A landfall anywhere south of us, especially Atlantic City or Sandy Hook, puts us in a situation where . . . there's no way for that water to come back out during the tidal cycles. So the water keeps rising."

Most fall storms that create problems for Long Island are nor'easters that don't form in the tropics. But seven late-season, unnamed hurricanes have struck the area since records began in the mid-1800s. They occurred in October or November of 1861, 1866, 1872, 1888, 1894, 1899 and 1944.